The demand for automobiles with electric propulsion is increasing due to the diminishing oil supply and concerns on Carbon emission to the atmosphere. Promising solutions including pure electric, hybrid electric or fuel cell based motor vehicles. Each of these systems is limited by the energy source that can supply power to the drive train. The following discussions are directed to hybrid electric vehicles, but one skilled in the art would recognize that this invention is also applicable to other vehicles such as pure electric or fuel celled based drive train.
Prior art energy source switching is controlled by a vehicle controller. Connection to the motor/motor controller from an energy source is made by turning on a switching element such as a contactor or a switching transistor. When an energy source is switched onto the motor/motor controller, the load presented to the energy source could be such that the in rush current could exceed the current limit of the switching element, effectively destroying it. It is for this reason a pre-charge circuit is used to ensure the voltage difference across the switching element is small and the in rush current is limited. However, when the energy source is being switched on with the motor controller engaged, the charge and/or discharge current are so high that effectively nullify the function of the pre-charge circuit and the voltage difference across the switching element remains large, and the switching on operation must be aborted and retried. It is also destructive when the energy source is being switched off with the motor controller engaged. As the switch was being opened and the switch contact resistance increased, the constant current demand forced the contact element to heat up rapidly, and as the switch opened, an electric arc is often formed across the open contact and destroyed the switch mechanism. It is for this reason most prior art vehicle controller design only allows source switching while vehicle is at rest, not during active drive cycles. It is desirable to have energy source switching in synchronization with motor controller. It allows deterministic engagement and disengagement of energy source to the power train, and remove the vehicle at rest constraint.
Super capacitors or batteries are often used as energy sources for electric propulsion. For vehicle designs intended to have substantive all-electric range, the energy storage source must store sufficient energy Kilowatt-hour (kWh) to satisfy the range requirement in real-world driving. The energy storage sources must be sized to provide adequate peak power (kW) for the vehicle to have a specified acceleration or hill climbing and top speed performance and the capability to meet appropriate drive cycles. These requirements will vary significantly depending on the vehicle drive train design, but they are reasonably straightforward to determine once the vehicle performance targets are established. However, real life road conditions often exceed the design target, and vehicle performance suffers. Since super capacitors can deliver peak power beyond even the most demanding driving conditions, it is a favored energy source for vehicle applications in this perspective. But super capacitors are expensive, and their energy volume density is low. One cannot pack sufficient super capacitors to sustain a long drive. On a 12-meter bus filled with super capacitors, the range is only 10 Kilometers, requiring many charge stations in the bus route. This approach is not popular, once the bus operator realized that single charge station failure would bring down the entire bus route operation. Super capacitors are more suitable for hybrid application with limited fuel saving capability. Super capacitor only systems are also known to run out of energy in the middle of a long hill climb. A super capacitor/battery dual source system are common, but the system often drains the super capacitor source too quickly, and the battery source does not have enough head room to handle peak energy requirement above the design limits.
Multiple energy sources are deployed for safety, flexibility, and reliability reasons. Only one of the energy sources is active at any time. Inactive energy sources can be charged by on board generators from either fuel cells, gasoline, or natural gas means, to extend the range of operation. When one energy source is depleted or damaged, another energy source is switched on, and the vehicle operation is resumed. Multiple energy sources could also be designed to enable fast swapping, further enhance the flexibility in operation. For multiple energy sources, each and every energy source must meet the peak power requirement individually, while collectively, or through fast swap means, can meet the driving range requirement. Not all energy sources need to be of the same element or chemistry composition. A super capacitor source based energy source can be paired with a battery source, or a power type battery source can be paired with that of the energy type. Prior art multiple source management is controlled by a vehicle controller. Decision for switching on or off an energy source is often based on the charge level of the energy source or State of the Charge (SOC). When SOC of the current source reaches a low limit, switch it off and switch on an energy source with higher SOC. Connection to the motor/motor controller of an energy source is made by turning on a switching element such as a contactor or a switching transistor. When an energy source is switched onto the motor/motor controller, the load presented to the energy source could be such that the in rush current could exceed the current limit of the switching element, effectively destroying it. It is for this reason a pre-charge circuit is used to ensure the voltage difference across the switching element is small and the in rush current is limited.
Precharge circuit, however, does not work when the motor/motor controller is engaged as described before. Failure to switch on an energy source during a drive cycle means the power is lost to the drive train. Sudden lost of power can lead to accident during a drive cycle. It is for this reason some prior art vehicle controller design only allows source switching while vehicle is at rest, not during active drive cycles. There are prior art super capacitor/battery multiple source systems that couple to the two with diodes rather than switches. Such architecture severely limits the operating voltage range of super capacitors and lowers the energy storage of super capacitors.
Battery or super capacitor based energy sources are a major cost item in an electric vehicle. It is desirable to meet the peak power requirement through the collaborative operations of multiple sources instead of designing each and every energy source to meet the peak power requirement. Collaborative operation means the energy sources can be scaled downwards and significant cost savings realized. It is also desirable to switch on/off energy source during the drive cycle without power interruption.